1. Field of the Invention
The invention relates to an air guide in a bonnet-covered vehicle engine bay, in particular in an agricultural tractor, where under the bonnet are located a combustion engine, components under high thermal load and low thermal load, and a fan which generates an air flow which is directed through air guide ducts past the combustion engine into the open.
2. Description of Related Art
Goods vehicles such as agricultural tractors are driven by water-cooled combustion engines which are located in an engine bay, covered by a bonnet, in front of the driver's console or cab. To cool the combustion engine, a cooling system comprising a radiator and a fan is located in front of the engine. Constant high drive power from the combustion engine leads to an increase in fuel consumption. Since for physical reasons only around one-third of the energy contained in the fuel is converted into mechanical useful power, the remaining energy occurs as waste heat. This is dissipated through the radiator to the environment by the coolant water of the combustion engine by means of the air flow generated by the fan. The air flow thus assumes temperatures between 90° and 100°.
In relation to components under high thermal load close to the combustion engine such as the exhaust turbocharger, exhaust pipe, silencer and exhaust gas recycling radiator, which are exposed to hot exhaust gases and can assume temperatures of 500° to 700°, the air flow leaving the radiator is however relatively cool. It is therefore conventional and known, for example from DE 10 2004 028 593 A1, to cool the components under high thermal load by passing an air flow leaving the radiator over these components. The air flow here, however, reaches a temperature level which cannot be tolerated without damage by components under low thermal load. In this earlier specification it is proposed to reduce the heat load of the components located in the engine bay, by driving the engine bay by means of a heat-screening component into several areas lying above each other but thermally separated, where components under high thermal load are arranged on the one side and components under low thermal load are arranged on the other side of the heat-screening component. According to this earlier specification the components under low thermal load are exposed to an air flow which is not branched from the air flowing through the radiator but bypasses the radiator without the effect of the fan. In agricultural tractors which often develop their maximum power at very low drive speeds or even when standing still, it is practically impossible to provide a screen with such a bypass as the installation space below the radiator is occupied extremely densely with other vehicle components; furthermore in these vehicles components under low thermal load such as electronic components, sensors and the starter or parts of the bonnet made of non-heat-resistant material are frequently located on the rear part of the combustion engine. There they are in some cases not only in direct contact with the hot housing of the combustion engine but are also exposed to the air flow which has heated up substantially while passing the combustion engine and the components under high thermal load. This can heat components under low thermal load unacceptably.
The object of the invention is, therefore, to create an air guide for the air flow from the fan below the vehicle bonnet which ensures no deterioration in function of heat-sensitive components.